Everything that is relevant; the whole thing. For example, He decided to take everything to college–his books, his stereo, his computer, his skis, the whole nine yards. The source of this expression is not known, but there are several possibilities: the amount of cloth required to make a complete suit of clothes; the fully set sails of a three-masted ship where each mast carries three yards, that is, spars, to support the sails; or the amount of cement (in cubic yards) contained in a cement mixer for a big construction job. (Colloquial).

Origin-

One of the most common theories of the phrase’s origin is that the expression dates from the Second World War, where “nine yards” was the length of an aircraft machine-gun ammunition belt, and to “go the full nine yards” was to use it up entirely. The expression, however, has been reliably dated back only to early 1967, in U.S. Air Force slang recorded in Vietnam. By November 1967 it was recorded in use in the U.S. Army, likewise from Vietnam, and by mid-1969 was appearing in newspaper advertisements in the United States.The first citation in the Oxford English Dictionary is from 1970, in the magazine Word Watching.-